Drug delivery systems of the brain, brain tumors, plus other tissues and tumors were studied. Single-and double-label and whole-body quantitative autoradiography were the major techniques employed. The rat tumor models examined included the ENU-induced oligodendroglioma, the RT-9 intracerebral and flank gliosarcomas, and the ASV-induced astrocytoma. For most small brain tumors, blood flow and transcapillary influx (the two major components of intravascular drug delivery) were similar to that of normal brain; however for large brain tumors, blood flow was generally reduced but transcapillary influx ranged from normal to greatly increased. Misonidazole delivery in the RT-9 flank tumor was mainly limited by blood flow. Intraperitoneal administration of test solutes yielded good delivery throughout the peritoneal cavity but lesser penetration into the intestine than the abdominal wall and diaphragm. Intra-arterial drug infusions achieve increased exposure of the target organ and decreased exposure of other tissues when drug is rapidly metabolized, blood flow through the infused artery is slow, and infused drug is rapidly broken-down in the target organ. The relative rates of drug transfer across the blood-brain barrier are not proportional to drug lipid solubility when octanol/water ratios exceed 1.0.